I understand that cracking is defined as the breaking down of large hydrocarbons into smaller ones, whereas dehydrogenation is the removal of hydrogen atoms

 

If propane were to be broken down into propene (C2H6 to C2H4), is that cracking or dehydrogenation, or both?

 

And I have a question that asks about cracking, in the diagram it shows liquid alkane on mineral fiber heated in a test tube with aluminum oxide. May I ask what is the aluminum oxide used for?

You anwsered yourself. Your two definitions are right so youve got the anwser.

 

With respect to the next bit, do you mean propane (C3H8) or ethane (C2H6). Either way you already know the anwser. When converting the alkane to the alkene youre removing the hydrogens not breaking the chain.

 

Aluminium oxide will be a catalyst for this reaction

If propane were to be broken down into propene (C2H6 to C2H4), is that cracking or dehydrogenation, or both?

 

Its a dehydrogenation.

 

 

If "cracking" is defined as "the breaking down of large hydrocarbons into smaller ones", then:

 

In the reaction you have listed, which is actually ethane to ethene and not propane to propene (that would be C3H8 to C3H6). You start off with a 2-carbon hydrocarbon and finish with a 2-carbon hydrocarbon which is not "smaller" as it has the same number of carbons. It is the number of carbon atoms which defines the size of the hydrocarbon, not the hydrogens.

So it seems cracking = breaking of C-C bonds in a hydrocarbon; dehydrogenation = breaking of C-H bonds.

Thanks for the answers

 

And pertaining to the propane (C2H6), my bad, meant to type (C3H8)

So it seems cracking = breaking of C-C bonds in a hydrocarbon; dehydrogenation = breaking of C-H bonds.

 

Sometimes Wiki is your friend http://en.wikipedia.org/wiki/Cracking_(chemistry)